All vehicles sold today, especially throughout the United States, must have occupant safety systems. Such systems include, for example, seatbelts and airbags. Much advancement has been made and continues to be made to improve the performance of the seatbelt and airbag systems. For example, much effort is being spent on studying the interaction of the seatbelt and airbag restraint systems on a vehicle occupant. Both systems exert restraining forces on a vehicle occupant's body during a collision. These restraining forces must be properly controlled in order to obtain the desired occupant protection. For example, with regard to the airbag system the pressure and forces exerted by the airbag on the vehicle occupant has been modified in the recent generation airbag systems to enhance occupant protection by matching the restraining force required to properly restrain the occupant during a vehicle collision to the particular occupant characteristics (size, weight, etc.) and the severity of the collision. Recent seatbelt systems have incorporated devices to further enhance the performance of the seatbelt restraint device. For example, many seatbelts today incorporate pretensioners which take up slack in a seatbelt the moment a vehicle collision is detected. The pretensioner and seatbelt retractor combination prevents unrestrained movement of the vehicle occupant during a vehicle collision. Other enhancements in seatbelts include “load limiters” which are incorporated into seatbelt retractors. The function of the load limiter is to pay out a predetermined length of seatbelt webbing in order to eliminate or reduce peak loads on a vehicle occupant. A load limiter is, for example, a torsion bar that is coupled to the spindle of the retractor and is configured to twist when a predetermined amount of torque is applied to an end of the torsion bar. As torque is applied to the torsion bar, the torsion bar twists and the spindle rotates a corresponding amount paying out the seatbelt webbing.
More recent enhancements in load limiters have been directed to addressing the differences in restraining forces required to safely bring occupants of different sizes to rest. For example, higher restraining forces will be applied to a larger vehicle occupant as opposed to a smaller vehicle occupant. Thus, the load limiting characteristics of torsion bars or other load limiting devices must be configured to accommodate different sized occupants. To this end, multi-stage load limiting devices have been developed. A multi-stage torsion bar, for example, is such a device. The multi-stage torsion bar is essentially two torsion bars that are axially aligned and joined at respective ends. The appropriate stage or portion of the torsion bar may be selectively oriented to provide the appropriate load limiting characteristics necessary to address the different sized occupants.
While these enhancements in safety restraints have achieved their intended purpose, further enhancements are needed to ensure appropriate occupant restraint is provided in the most cost effective and efficient manner.